Feeding of mass fish species by natural and farmed juveniles of pacific salmons on example of the Obutonai River (Sakhalin Island)

Abstract

Feeding of mass fish species caught in the Obutonai River (Sakhalin, Nevelsk district) was examined in May-July 2021, in the time of downstream migration of natural pink salmon Oncorhynchus gorbuscha juveniles and release of chum salmon O. keta juveniles from a fish hatchery. Three fish species capable to predate on salmon juveniles were presented in the catches in significant number, as whitespotted char Salvelinus leucomaenis, pacific redfin Tribolodon brandtii, and juveniles and dwarf males of masu salmon O. masou. These species occurred in different lower reaches of the river. Whitespotted char fed by both pink and chum juveniles, but was relatively few in number (single mass catch). Two-year-olds and dwarf males of masu salmon fed less intensively and pacific redfin fed exceptionally rarely on the migrating pink salmon juveniles of natural origin, but both these species consumed actively the chum juveniles released from a hatchery. The masu salmon juveniles had the maximum abundance in early June, then their number decreased. The pacific redfin abundance had complicated dynamics because of migrations both from the upper part of the river and from the sea; pacific redfin spawned in late June when ate exclusively eggs of the same species.